The present invention relates to a radio communication system and, particularly, a radio communication system in which a plurality of communication groups each having a master station and at least one slave station use a single common carrier frequency.
For example, in the so-called paging system in which data is sent from a master station to a slave station unidirectionally, it is impossible to know, at the master station, whether or not the data is received by the slave station. Therefore, a system for sending a data receipt acknowledgment signal from the slave station has been proposed. Such a system, however, would operate smoothly only when each group uses one or more frequencies which are different from those of other groups. That is, when all of the groups use one frequency commonly, the problem of signal collision is unavoidable.
The signal collision problem may include the collision between signals from different master stations and a collision between signals from a slave station of one group and a master station of another group, etc. In order to avoid such signal collisions, it is possible for one station to detect a carrier frequency from another station and transmit a signal during a time in which the carrier frequency is not used by the other station. In such a system, however, both the master station and the slave station must have carrier sense functions, respectively. Further, in such a case, it is impossible, at the side of the master station, to know when it can expect to receive a reception acknowledging signal from the slave station. Further, since it is generally difficult to determine, at the side of the slave station, where the slave station itself is located in the service area of the master station to which it belongs and there may be a case where the master station can not sense the carrier from the slave station.